Typically when there are periodic disconnections from a Synology, the solution is to disable the default setting to allow disk sleep on the storage.
We recommend disabling disk sleep for the workstation and target. For a SAN, there is no mechanism for the workstation to tell the storage that it is sleeping (or vice versa), so from the perspective of the storage, the client has disconnected, often without a clean unmount. This mismatch can also lead to application crashing and/or filesystem corruption.
Here are the default settings from DiskStation Manager (DSM 5.1-5055) for a model DS214se.
From Control Panel > Hardware & Power > HDD Hibernation, the hibernation should be set to "none" in order to disable sleep.
